There are widely used incandescent light bulbs which produce light with a filament such as tungsten filament heated by an electric current flown through it. However, although incandescent light bulbs show high electric power-to-light conversion efficiency (80% or higher), much of the light thereby produced consists of infrared radiation components as shown in FIG. 6 (90% or more in the case of 3000K shown in FIG. 6), and therefore the electric power-to-visible light conversion efficiency thereof is low. Specifically, visible light conversion efficiency of incandescent light bulbs is as low as about 15 lm/W (visible light conversion efficiency of fluorescent lamps is 90 lm/W). In addition, although incandescent light bulbs show a radiation spectrum close to sunlight providing superior color rendering properties, they have a problem that they impose large environmental loads.
Moreover, it is well known that if grain size of metallic materials including tungsten becomes large due to recrystallization, strength and ductility thereof decrease. Specifically, recrystallized grains of pure tungsten have an equi-axed crystal structure and a relatively round shape, and contain many grain boundaries perpendicular to a line axis. Therefore, if a filament coil made of pure tungsten is used at a high temperature, slippage occurs at crystal grain boundaries extending along the radial direction of the filament, and the filament is easily deformed with a small external force such as own weight (creep deformation). Therefore, the filament comes to be easily locally heated, and easily cause disconnection.
In order to obtain small crystal grains (grain boundary strengthening), tungsten metals added with various elements and compounds (doped tungsten) are practically used. For example, Patent document 1 proposes a filament using tungsten added with thoria (ThO2) or tungsten added with Re. In addition, tungsten added with La2O3, CeO2, or potassium (K) for the grain boundary strengthening is marketed. In doped tungsten added with a trace amount of thoria or potassium (K), growth of crystal grains along the radial direction of the filament is suppressed, and therefore recrystallized grains thereof are long and large crystals extending along the processing direction (filament axis direction). Thoria dispersedly exists at crystal boundaries of tungsten to prevent migration of the grain boundaries, and thereby suppresses growth of grains to provide small recrystallized grains. Potassium (K) suppresses growth of grain boundaries along the radial direction of the filament to provide long and large crystals extending along the processing direction.
Further, Patent document 2 proposes use of tungsten having a purity of 4N (99.99% or higher) for an anode and use of tungsten added with K as a cathode in a high pressure mercury lamp for preventing impurities contained in tungsten from evaporating and adhering to internal wall of an arc tube to cause blackening.